Since penicillin discovered by Fleming was successfully put into industrial production through the cooperation between microbiologists and chemical engineers in 1945, Submerged fermentation technique has opened a modem fermentation industry. Solid state fermentation has not fulfilled the requirement of modem fermentation industry and has thus been ignored because it has no engineering means to solve the problems such as transportation, agitation, oxygen supply and control of temperature, humidity and pH. The key point is there has not been a good solid state fermentor meeting the requirements of modem fermentation industry, which remains a world-wide difficulty.
The strict requirement on microbe pure cultivation and large scale production imposed by modem fermentation industry makes liquid submerged fermentation occupy a dominating position. Solid state fermentation technology has been regarded as old and backward because its process and equipment are subject to bacteria pollution, its fermentation conditions are difficult to control and its industrial scaling up is difficult. Solid state fermentation, however, has many advantages, for example, simple and short flow process, wide availability of raw materials, low energy consumption, low cost and no pollution; it is, therefore, very attractive and potential in the development of bioreactors. In order to change the backwardness of solid state fermentation industry, for half a century, especially since the appearance of biochemical engineering in the seventies of the 20th century, there has been no lack of people who attempted to solve this difficult problem by proposing various means, B. K. Lonsanc summarized them into nine types (1) drum type, (2) wooden box type, (3)capped plate type, (4) vertical cultivation box type (5) inclined culturing box type, (6) tray type, (7)belt conveyor type, (8) cylinder type (9) mixed type, K. E. Aidou divided the solid state fermentation apparatus into ten types, similar with those proposed by B. K. Lonsanc. They can be summarized into two categories, namely static and dynamic according to the state of culture medium. Static state means motionless culture medium, which makes mass transfer, heat transfer, oxygen supply and control of temperature, humidity and pH difficult. Dynamic state means that the culture medium is in intermittent and continuous motion, which significantly improves mass transfer, heat transfer and oxygen supply, but the mechanical parts used are unfavorable to aseptic operation, energy consumption on material agitating is high, mycelia are likely to be damaged, and engineering scaling up is difficult.
On one hand, people try to improve the traditional solid state fermentors, such as bend trays, bend boxes and bend pools, mainly mechanizing physical labor involved and strengthening ventilation. On the other hand, people actively develop closed type solid state fermentor with mechanical agitation suitable for microbe pure cultivation, among others, the most studied is the drum type, and there are rotating tray or rotating rake type, belt conveyor type, mechanical lifting type and the like. Because of the reasons such as complicated driving mechanism, no easy strict sealing and dead space being not readily cleaned, solid state fermentors of such types have not met the practical requirement imposed on pure cultivation and industrial scaling up by modem fermentation industry.
The solid state fermentors used in production nowadays are still the traditional tray type and deep layer ventilation pond types, as well as the practice of placing culture medium pressed as cakes or square bricks in rooms. A larger scale method uses tunnel kiln continuous tray fermentation. All the types described above are static and used in the production process of natural fermentation. From the view point of the “three transfers and one reactor” chemical engineering theory, the direction of development should be dynamic fermators for intensifying mass and heat transfer. Dynamic fermentation reactors of closed type have, therefore, been the hot point in research in China and other countries. Particularly, drum type solid state fermentation reactors are reported in many literatures to have gone into an industrial pilot test stage. They did not endure tests of long time production because of being difficult to prevent pollution with miscellaneous bacteria. Closed dynamic fermentation reactors of other types are still limited to being in laboratory research.